This invention relates to techniques for the optical measurement of thin films. More particularly, this relates to measurement of optical properties of either solid or fluid thin films by measuring the optical interaction of laser light from an injection laser diode in which the thin film is adjacent to one of the facets of the laser diode.
There are a variety of different ways for measuring the optical properties of a thin film. The most usual property to be measured is thickness. Techniques at present include quartz crystal microbalance, witness, optical monitoring, and electroluminescence. In the quartz crystal microbalance technique, a quartz crystal is placed in the chamber in which the thin film is deposited. The quartz crystal has a particular resonant frequency depending upon the mass of the crystal. As more material is deposited upon the crystal, the frequency of the crystal will change. Thickness can be derived from the change in frequency. In the witness, usually one or more glass flats is exposed to deposition within the deposition chamber for the optical thin film. The glass is then removed from the chamber for measurement. The optical monitoring technique is somewhat related to the witness technique. In optical monitoring a beam of light from a remote object source is directed upon a test flat within the deposition chamber. The reflected/transmitted light from the flat is then measured to determine the thickness of the deposited film. In the electroluminescence technique, a quite different process is used. In a system marketed by the INFICON Company, the mass of the deposited material is determined in its gaseous state prior to deposition within the chamber. The mass is measured by running a current through the gas and measuring the resulting luminescence of the gas. The luminescence measurement in turn is a measurement of the mass of the gas within the chamber. The deposition rate can then be calculated and this in turn will give the thickness of the deposited film. All of these techniques suffer from one or more deficiencies. Some have relative low accuracy in measuring the desired optical property of the thin film. Others require that the deposition process be stopped to make the measurement. And others utilize equipment which is very expensive.